Adenoviruses have been widely studied as infectious agents, as a subject for basic research, and for their potential use in gene therapy and vaccines. Forty-nine human adenoviral serotypes have been identified and they are categorized into six subgenera (A through F) based on nucleic acid comparisons, fiber protein characteristics, and biological properties. For example, group A includes serotypes 12 and 31, group B includes serotypes 3 and 7, group C includes serotypes 2 and 5, group D includes serotypes 8 and 30, group E includes serotype 4, and group F includes serotypes 40 and 41.
In terms of general structure, all adenoviruses examined to date are nonenveloped, regular icosahedrons of about 80 nanometers in diameter. Adenoviruses contain linear, double-stranded DNA that is complexed with core proteins and surrounded by the adenoviral capsid. Individual virions contain about 11 different proteins designated by Roman numerals (II-XII), in order of their decreasing size on SDS gels.
The capsid is composed of seven structural proteins: II (hexon), III (penton), IIIa, IV (fiber), VI, VII, and IX. The capsid comprises 252 capsomeres, of which 240 are hexon capsomeres and 12 are penton capsomeres. Hexon capsomeres, which are trimers of the hexon protein, make up about 75% of the protein of the capsid. Penton capsomeres, which are pentamers of the penton protein, are situated at each of the 12 vertices of the virion. Each penton capsomer is bound to six adjacent hexon capsomeres and a fiber. The fiber, which is usually a trimer of the fiber protein, projects from the penton capsomer. The hexon protein and, to a lesser extent, the fiber protein comprise the main antigenic determinants of an adenovirus and also determine serotype specificity.
Researchers have examined and compared the structure of the capsid proteins of different adenoviral serotypes, and in particular hexon proteins, in an effort to define the regions of the proteins against which neutralizing antibodies are elicited. The predominant regions in hexon protein against which neutralizing antibodies are directed appear to be in loops 1 and 2 (i.e., LI or l1, and LII or l2, respectively), which project outward from the base of the hexon capsomere. Analysis of loops 1 and 2 from different adenovirus hexon proteins has revealed the presence of seven discrete hypervariable regions (HVR1 to HVR7) corresponding to locations where the hexon proteins differ considerably between serotypes.
The core of an adenovirus virion contains the linear double-stranded DNA genome and associated proteins V, VII, X (mu), IVa2, and terminal protein (TP). The genome organization of different adenoviruses is conserved and has been proposed to have a timing function, wherein the ends of the genome are transcribed first (the immediate early genes E1 and E4 are located at opposite ends of the linear genome). Early transcription of E1 and E4 leads to the opening of the central region of the genome, allowing transcription of the central region.
Adenoviral genomes typically comprise eight RNA polymerase II transcriptional units: five early units, E1A, E1B, E2A-E2B, E3, and E4; two delayed early units, IX and IVa2; and the Major Late transcriptional unit. The Major Late transcriptional unit is further subdivided into L1-L5 regions based upon the use of alternative splicing sites. The transcriptional units often express proteins of similar function. For example, the E1A unit codes for two proteins responsible for activation of transcription and induction of S-phase upon cellular infection; the E1B transcription unit encodes two proteins that inhibit cellular apoptosis; the E3 transcriptional unit is involved in evasion of the immune response; and the Major Late transcriptional unit encodes structural proteins necessary for assembly of the capsid.
For the purpose of gene therapy and vaccination, recombinant adenoviral vectors have been designed to encode and express heterologous genes and antigens. The Ad2 and Ad5 serotypes have been used most extensively in this context. Heterologous sequences have been inserted into the adenoviral genomes, including in the early transcriptional units and in the coding regions of various structural proteins, such as hexon, penton, and fiber. In many cases, deletions in the adenoviral genome (e.g., in the E1 regions) have been used to create replication-defective adenoviral vectors, which have generally been considered safer for administration to human subjects. Despite such extensive research and development, there remains a need in the art for new recombinant adenoviral vectors suitable, for example, as vaccines for infectious diseases.